The printed circuit board (PCB) is an electrical connection carrier of the electronic components. The PCB can be categorized as a rigid printed circuit board (RPC), a flexible printed circuit board (FPC), or a rigid-flex printed circuit board (RFPC). The PCB can be structurally categorized as a single-sided printed circuit board, a double-sided printed circuit board, or a multilayer printed circuit board (more than 3 layers). The rigid printed circuit board has characteristics of heat resistance, no deformation and better planeness. The flexible printed circuit board has an advantage of a high flexibility. The rigid-flex printed circuit board has functions and characteristics of both the rigid printed circuit board and the flexible printed circuit board, and it is widely used in the communication control module (CCM), and the camera module of devices such as a notebook computer, a tablet computer, a smart mobile phone, a wearable electronic bracelet, etc. The single-sided printed circuit board is widely used in the hard disc drives (HDD), optical pickup heads, etc., due to the characteristics of high flexibility and low cost. The circuit design of the double-sided printed circuit board is more complicated than that of the single-sided printed circuit board, and the thickness of the double-sided printed circuit board is also slightly more. A multilayer printed circuit board having more than three layers is more flexible in circuit design due to more layers.
Now a days, electronic products, such as a liquid crystal display (LCD), a plasma display panel (PDP), a chip on flex (COF), etc., require thin PCB laminates that can provide high density, high dimensional stability, high temperature resistance and reliability of the circuits. As the electronic products are becoming thinner and smaller, the thinner metal layer and dielectric layer are required in the print circuit board. The ultra-thin adhesive free circuit board will become a mainstream of the market, which gradually replaces the three layers (dielectric layer/adhesive layer/copper foil) adhesive laminate. Ultra-thin (smaller than 5 microns) circuit board needs a thinner copper foil. However, the thinnest copper foil existing in the current industry has a thickness of about 12 microns, and thus, it is hard to manufacture the ultra-thin circuit board. Also, if the copper foil is too thin, then it has low mechanical strength, so it is difficult to conduct the copper coating. A method to prepare an ultra-thin adhesive free circuit board is to conduct chemical copper plating or sputtering copper plating directly on the dielectric layer. However, due to smooth surface and poor hydrophilicity (a low surface energy) of the dielectric layer (polyimide, epoxy resin and etc.), the deposited copper film gets easily peeled off from the dielectric layer. It is necessary to conduct a modification treatment on the surface of the dielectric layer, so as to provide a good binding force between the dielectric layer and the deposited copper film. The peeling strength is required to be more than 7N/cm (industry standard).
In the recent ten years, researches on surface modification treatment of the dielectric material, polyimide film (PI), of the ultra-thin printed circuit board are actively carried out internationally. The main treatment methods include acid and alkali treatment method, plasma treatment method, ion beam treatment method, and surface grafting method.
In 1988, Ruoff et al. used the reactive oxygen ion beam to etch PI film, and to improve and measure adhesion between the PI film and copper by adjusting different energies, ion current density, etching time and other factors (Improvement of adhesion of copper on polyimide by reactive ion-bean etching [J]. IBM Journal of Research and Development, 1988, 32:5). The adhesive strength of the treated PI film, under optimal condition, was 6.9N/cm. Compared to the untreated PI film, though the adhesive strength of the treated PI film is increased nearly 25 times, it still fails to meet the standard of use. In 2006, Ju Hi Hong et al. used PSII (Plasma source ion implantation) technology to deposit copper film on the surface of the PI film. The results showed that the binding force between the copper film and the polyimide film was greatly increased, but it still cannot meet the requirement of use (Improvement of adhesion properties for Cu films on the polyimide by plasma source ion implantation [J]. Surface & Coatings Technology. 2006, 201:197). The patent application CN101684554A disclosed a chemical copper plating solution for a polyimide film and a method of surface chemical copper plating thereof, but it also failed to address the key problem of the binding force between the polyimide film and the copper film, and thus failed to break through the technical bottleneck of preparing the ultra-thin adhesive free printed circuit board laminate. The patent application CN102196904A disclosed a laminate and a manufacturing method thereof. The invention partially modified the surface of the resin film having a thermoplastic cyclic olefin resin by ionizing radiation.